We report a practical superconducting terahertz (THz) source, comprising a stack of Bi2Sr2CaCu2O8intrinsic Josephson junctions (IJJs) and a vanadium dioxide (VO2) tunable attenuator with coplanar interdigital contacts. The electrical triggering phase transitions are observed not only at room temperature, but also at low temperatures, which provides a proof of the electrical triggering. Applying this, the VO2attenuator is implemented for the independent regulations on the emission powers from the IJJ THz emitter, remaining frequencies and temperatures unchanged. The attenuation can be tuned smoothly and continuously within a couple of volts among which the maximum is, respectively, −5.6 dB at 20 K or −4.3 dB at 25 K. Such a power-adjustable radiation source, including the VO2attenuator, can further expand its practicability in cryogenic THz systems, like superconducting THz spectrometers.